I. Field of the Invention
The present invention relates generally to resistive ladder switching matrices. More particularly, the present invention relates to resistive ladder switching matrices used for operating keypads in portable electronic devices such as, for example, mobile telephones. Even more particularly, the present invention relates to a method and apparatus for calibrating resistive ladder switching matrices so as to compensate for variations in operating parameters such as temperature and oxidation on the keypad contacts connecting the keypad resistive ladder to the processing circuitry in the portable electronic device.
II. Description of the Related Art
Resistive ladder switching matrices (or networks) are well known in the art, and are typically used for operating keypads in electronic devices. In such systems, depression of a button on the keypad completes the electrical connection of one or more resistors between a reference voltage and ground such that a unique voltage is produced at the output of the matrices in response to the depression of each button on the keypad. By monitoring the amplitudes of the voltage signals output by the ladder network, a microprocessor can determine which button a user has depressed on the keypad.
Since the amplitudes of the output signals output by a ladder network are used to differentiate between depression of different buttons on the keypad, it is important that the network be properly calibrated so that the microprocessor can properly associate each voltage output from the network with the correct button on the keypad. Unfortunately, changes in temperature and oxidation on the contacts of the resistive ladder network will cause the voltage output in response to depression of each individual button to shift. Unless corrections are otherwise made to compensate for these voltage shifts, the microprocessor will be unable to correctly determine which button a user has depressed. Thus, it would be desirable to have system that could be used to calibrate the ladder network so as to account for such voltage shifts and insure that the microprocessor correctly associates each output voltage from the ladder network with the appropriate button on the keypad.
Electronic devices such as certain cellular telephones (known as `flip-phones`) and laptop computers have moveable covers that are opened and closed. In the case of flip-phones, the keypad used with the resistive ladder network may itself be positioned on the moveable cover such that keypad will be exposed and active when the cover is closed and "hidden" and inactive when the cover is in its open position. It would be particularly desirable if there were a system that could be used for calibrating resistive ladder networks which was adaptable for use with electronic devices such as flip-phones and laptop computers having moveable covers.
These problems and deficiencies are recognized and solved by the present invention in the manner described below.